Dualism consists of substances, which include corporeal things and thinking things. The essence of the mind is thought be the essence of the body but its extension. Human bodies and their properties are objects of sense perception. Minds and their properties cannot be objects of sense perception. Interaction between mind and body is rationally unintelligible; in a human being, a mind and a body are substantially united.
Res Cogitans is Latin for =……the Meditator advances the claim that he is a thing that thinks, an argument called the sum res cogitans, after its Latin formulation. There are three controversies regarding the claim "I am...in the strict sense only a thing that thinks," which we will examine in turn: whether the claim is metaphysical or epistemological, what is meant by "thing," and what is meant by "thinking." It is more plausible to read the sum res cogitans as an epistemological remark, saying that, "whatever else I may be, I know only that I am a thing that thinks." However, in some of his writings, Descartes makes it plausible to read him as making a metaphysical remark, that "I am only a thing that thinks." His reasoning might go something like this: "I know that I am a thinking thing, and I do not know whether I am a bodily thing. My body and my mind cannot be one and the same, because I should either know both of them or know neither of them. Since I know I am a thinking thing, and know that my body and my mind are two separate things, I can conclude that I am not a bodily thing. Therefore, I am only a thing that thinks." In so arguing, however, Descartes would commit the so-called "intentional fallacy" of basing an argument on what one does not know. If two things had to be either both known or both not known in order to be identical, we could argue that Bruce Wayne and Batman are not one and the same as well. "Thing that thinks" also carries some ambiguous baggage. By "thing," Descartes could simply be using the word as we do today, as an ambiguous throwaway word when we don't want to be more specific. More likely, though, he is using it to mean substance, the fundamental and indivisible elements of Cartesian ontology. In this ontology, there are extended things (bodies) and thinking things (minds), and Descartes is here asserting that we are minds rather than bodies. Of course, "thinking" is also highly questionable. Does Descartes mean only the intellection and understanding that is characteristic of the Aristotelian conception of mind? Or does he also include sensory perception, imagination, willing, and so on? At the beginning of the Second Meditation, the Meditator has cast sensory perception and so on into doubt, but by the end of the Second Meditation, sensing, imagining, willing, and so on are included as attributes of the mind. This question is further explored in the commentary on the next section.
The Meditator tries to clarify precisely what this "I" is, this "thing that thinks." He concludes that he is not only something that thinks, understands, and wills, but is also something that imagines and senses. After all, he may be dreaming or deceived by an evil demon, but he can still imagine things and he still seems to hear and see things. His sensory perceptions may not be veridical, but they are certainly a part of the same mind that thinks.
The Meditator then moves on to ask how he comes to know of this "I." The senses, as we have seen, cannot be trusted. Similarly, he...